STTM devices are non-volatile memory devices that utilize a phenomenon known as tunnelling magnetoresistance (TMR). For a structure including two ferromagnetic layers separated by a thin insulating tunnel layer, it is more likely that electrons will tunnel through the tunnel layer when magnetizations of the two magnetic layers are in a parallel orientation than if they are not (non-parallel or antiparallel orientation). As such, an MTJ can be switched between two states of electrical resistance, one state having a low resistance and one state with a high resistance. The greater the differential in resistance, the higher the TMR ratio (RAP−RP/RP*100 where RP and RAP are resistances for parallel and antiparallel alignment of the magnetizations, respectively) and the more readily a bit can be reliably stored in association with the MTJ resistive state. The TMR ratio of a given MTJ is therefore an important performance metric of an STTM.
For an STTM device, current-induced magnetization switching is used to set the bit states. Polarization states of one ferromagnetic layer are switched relative to a fixed polarization of the second ferromagnetic layer via the spin transfer torque phenomenon, enabling states of the MTJ to be set by application of current. Upon passing a current through the fixed magnetic layer, angular momentum (spin) of the electrons is polarized along the direction of the magnetization of the fixed layer. These spin polarized electrons transfer their spin angular momentum to the magnetization of the free layer and cause it to precess. As such, the magnetization of the free magnetic layer can be switched by a pulse of current (e.g., in about 1 nano-second) exceeding a certain critical value with magnetization of the fixed magnetic layer remains unchanged as long as the current pulse is below a higher threshold attributable to a different geometry, an adjacent pinning layer, different coercivity (Hc), etc.
MTJs with magnetic electrodes having a perpendicular (out of plane of substrate) magnetic easy axis have a potential for realizing higher density memory than in-plane variants. Generally, perpendicular magnetic anisotropy (PMA) can been achieved in the free magnetic layer in the presence of interfacial perpendicular anisotropy established by an adjacent layer, such as MgO, when free magnetic layer is sufficiently thin. This structure however is associated with greater thermal instability, which can significantly shorten the non-volatile lifetime of a memory element. PMA can also be achieved through coupling to a strongly perpendicular film disposed adjacent to the free layer. While thermal stability is improved with this structure, TMR ratio tends to be low due to crystal mismatch.
Perpendicular MTJ structures and formation techniques capable of achieving both a high TMR ratio and good thermal stability are therefore advantageous.